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Full text of "Evaluation of the efficacy of 3% citronella candles and 5% citronella incense for protection against field populations of Aedes mosquitoes."

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Journal of the American Mosquito Control Association, 12(2):293-294, 1996 
Copyright © 1996 by the American Mosquito Control Association, Inc. 





Department of Environmental Biology, University of Guelph, 
Guelph, Ontario NIG 2WI Canada 

ABSTRACT. We assessed the efficacy of 3% citronella candles and 5% citronella incense in protecting 
subjects from bites of Aedes spp. under field conditions. The study was conducted in a deciduous woodlot 
in Guelph, Ontario, Canada from July 26 to August 10, 1995. Eight subjects, dressed identically, were 
assigned to one of 8 positions on a grid within the study area. Two citronella candles, 2 citronella incense, 
2 plain unscented candles, or no candles (i.e., nontreated controls) were assigned to 2 positions on the 
grid each evening. Subjects conducted 5-min biting counts at each position and performed 16 biting 
counts per evening. On average, subjects received 6.2 ± 0.4, 8.2 ± 0.5, 8.2 ± 0.4, and 10.8 ± 0.5 bites/ 
5 min at positions with citronella candles, citronella incense, plain candles, and no candles, respectively. 
Although significantly fewer bites were received by subjects at positions with citronella candles and 
incense than at nontreated locations, the overall reduction in bites provided by the citronella candles and 
incense was only 42.3 and 24.2%, respectively. 

Candles containing oil of citronella are sold 
commercially in the United States and Canada, 
with some manufacturers claiming that these 
products "reduce the annoyance of biting in- 
sects," specifically mosquitoes. However, with 
the exception of unpublished data cited in Curtis 
(1986) that suggested that citronella candles do 
not appreciably reduce the biting rates of mos- 
quitoes, quantitative studies of the repellency 
provided by citronella-based candles or incense 
are lacking. The purpose of this study was to 
assess the efficacy of commercially available 3% 
citronella candles and 5% citronella incense to 
protect people from bites of Aedes spp. mosqui- 
toes under field conditions. 

The study was conducted in a deciduous 
woodlot in Guelph, Ontario (43°45'N, 80°20'W). 
This woodlot, which is surrounded by large 
snowmelt pools where large numbers of imma- 
ture Aedes spp. develop has been described pre- 
viously (Surgeoner and Heal 1992, Heal et al. 
1995). Eight subjects (4 ? $; 4 3 6) were used 
in this study, which was conducted on 8 nights 
(July 28, 31 and August 1, 2, 7, 8, 9, 10, 1995). 
An additional person timed biting counts and re- 
corded weather conditions. The subjects wore 
headnets, white cotton gloves, and green cov- 
eralls with the sleeves rolled up to the elbows. 
They were not allowed to wear repellent. 

Biting counts were initiated by 1900 h each 
night. The subjects were assigned to one of 8 
positions on a grid within the study area at the 
beginning of the evening and rotated through all 
8 positions twice each night. Positions were sep- 
arated by at least 10 m. Each treatment was 
placed at 2 positions on the grid each evening. 

The treatments were citronella candles, citronella 
incense, plain unscented candles, and no candles 
(i.e., nontreated controls). Two citronella candles, 
2 plain candles, or 2 citronella incense were 
placed at each treated position on top of 35-cm 
plastic stands 1 m apart. A plastic lawn chair was 
placed between the plastic stands and subjects 
conducted biting counts while seated on the lawn 
chairs. Treatments were assigned to positions on 
the grid such that each treatment was at each po- 
sition twice during the 8-night evaluation. 

Biting counts were made during 5-min peri- 
ods at each position. The subjects aspirated all 
of the mosquitoes biting both exposed forearms 
into 150-ml clear plastic vials. The subjects re- 
corded the number of mosquitoes captured and 
then moved to the next position on the grid. 
Thus, subjects made 4 biting counts at each 
treatment, each night. Mosquitoes collected 
were pooled each night and 30/night were ran- 
domly selected and identified using keys of 
Wood et al. (1979). Ambient temperature, rela- 
tive humidity, wind speed, and wind direction 
within the study site were recorded at =30 min 
intervals during the biting counts. 

The data were analyzed using a 4-factor anal- 
ysis of variance with subject, treatment, position 
on the grid, and night as the independent vari- 
ables. Because the number of mosquitoes col- 
lected per 5-min biting count varied widely be- 
tween subjects, and the variance was correlated 
with the mean, these data were log 10 transformed 
prior to the analysis. Subject, position on the 
grid, and night were random variables and treat- 
ment was a fixed variable. The analyses were 
completed using Statistical Analysis Systems 



Journal of the American Mosquito Control Association 

Vol. 12, No. 2 

Table 1. Mean number ± SE of mosquitoes collected per 5-min biting count (n = 128) at 
positions with 3% citronella candles, 5% citronella incense, plain candles, or nontreated controls. 






Mean ± SE 1 
Percent reduction 2 

6.2 ± 0.4A 

1.2 ± 0.5B 

8.2 ± 0.4B 

No treatment 
10.8 ± 0.5C 

1 Means followed by the same letter are not different at P < 0.05; Student-Newman-Keuls test. 

2 Percent reduction calculated as: [(no. biting subjects at nontreated positions - no. biting at treated positionsWno. biting at 
nontreated positions)] X 100. 

version 6.04 (SAS Institute Inc., Cary, NC). Per- 
cent reduction in the number of bites provided 
by each treatment was also calculated as: [(no. 
biting subjects at nontreated positions - no. bit- 
ing at treated positions)/(no. biting at nontreated 
positions)] X 100. 

Each night 800-1,300 mosquitoes were col- 
lected. Ten mosquito species were identified from 
240 individuals subsampled throughout the study. 
The species and percent composition were: Aedes 
euedes Howard, Dyar and Knab (59.2%), Ae. 
vexans (Meigen) (12.5%), Ae. fitchii (Felt and 
Young) (11.2%), Ae. trivittatus (Coquillett) 
(10.4%), Ae. excrucians (Walker) (2.5%), Ae. 
canadensis (Theobald) (2.1%), Ae. stimulans 
(Walker) (0.8%), Ae. implicates Vockeroth 
(0.4%), Coquillettidia perturbans (Walker) 
(0.4%), and Anopheles quadrimaculatus Say 
(0.4%). Ambient air temperatures ranged from 19 
to 26°C during biting count evaluations. Wind was 
less than 5 km/h during all nights except for <30 
min on August 1 when gusts up to 8 km/h were 
recorded. Regardless of the wind direction, the 
subjects were always exposed to the smoke of at 
least one candle or incense. Light rain fell for < 10 
min on August 1; however, it did not rain on any 
other evening. None of the meteorological param- 
eters measured would have deterred host-seeking 
activity by mosquitoes and therefore did not affect 
the results obtained. 

Subjects received significantly fewer bites (P 
< 0.001) at positions with citronella candles 
compared with the other 3 treatments (Table 1 ). 
Significantly more mosquitoes (P < 0.001) were 
collected at nontreated positions than ones with 
citronella incense and plain candles and biting 
counts did not differ significantly (P > 0.5) be- 
tween these 2 treatments (Table 1). Although 
significantly fewer mosquitoes were collected at 
treated positions compared with nontreated ones, 
the overall percent reduction provided by the cit- 
ronella candles, citronella incense, and plain 
candles was only 42.3, 24.2, and 23.1%, respec- 
tively. Surprisingly, plain candles reduced the 
biting activity of mosquitoes, presumably be- 
cause the light, heat, carbon dioxide, and mois- 

ture produced drew mosquitoes away from the 
subjects. Considering the reduction in mosquito 
biting activity produced by the act of burning 
candles, the addition of 3% citronella to candles 
further decreased biting activity by only 19.2%. 

As in previous studies with woodsmoke (Snow 
et al. 1987), citronella candles or incense were 
ineffective for reducing the biting pressure of 
mosquitoes, and their use by the general public 
should be discouraged. Although one manufac- 
turer (S.C. Johnson & Son, Limited, Brantford, 
Ontario) specified that the candles should be 
placed =45 cm apart, for practical and safety rea- 
sons we spaced the candles and incense 1 m 
apart. In general the directions for use of these 
products are vague or nonexistent and consumers 
are not explicitly told how many candles must be 
used to produce adequate levels of protection 
from mosquitoes. Increasing the number of can- 
dles or incense per unit area may have increased 
the efficacy of these products; however, it is un- 
likely that consumers would tolerate the exces- 
sive amounts of smoke and odor that would have 
to be generated to produce this effect. 

We thank the Ontario Ministry of Agriculture 
and Food and Rural Affairs for continued in- 
frastructural support. We also thank the subjects 
who participated in this study. 


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